Method and apparatus for detecting contention during random access procedure in a mobile communication system

ABSTRACT

A method is provided for performing a random access procedure in a mobile communication system. The method includes receiving a random access response message from a Base Station (BS); transmitting a message including a Cell-Radio Network Temporary Identifier (C-RNTI) to the BS, if the C-RNTI exists; and determining that a contention for the random access procedure is resolved successfully, when User Equipment (UE)-specific control information addressable by the C-RNTI is received from the BS.

PRIORITY

This application is a Continuation of U.S. application Ser. No.12/028,508, which was filed in the U.S. Patent and Trademark Office onFeb. 8, 2008, and claims priority under 35 U.S.C. §119(a) to KoreanPatent Application Serial No. 2007-14024, which was filed in the KoreanIntellectual Property Office on Feb. 9, 2007, the content of each ofwhich is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a mobile communicationsystem, and more particularly, to a method and apparatus for efficientlydetecting contention, for reducing signaling overhead during a randomaccess procedure.

2. Description of the Related Art

The Universal Mobile Telecommunications System (UMTS) is a 3^(rd)Generation (3G) asynchronous mobile communication system operating inWideband Code Division Multiple Access (WCDMA), based on European mobilecommunication systems, Global System for Mobile Communications (GSM) andGeneral Packet Radio Services (GPRS). The 3^(rd) Generation PartnershipProject (3GPP) that standardized UMTS is now discussing Long TermEvolution (LTE) as the next generation of UMTS, the Evolved UMTS. The3GPP LTE is a technology for enabling packet communications at or above100 Mbps, aiming at commercialization by 2010. For deploying the LTEsystem, many communication schemes have been proposed. Among them, thereare schemes of reducing the number of nodes on a communication line bysimplifying a network configuration or of optimizing radio protocols forradio channels.

FIG. 1 is an exemplary view illustrating an Evolved 3GPP LTE system.

Referring to FIG. 1, each of Evolved UMTS Radio Access Networks(E-UTRANs or E-RANs) 110 and 112 is simplified to a 2-node structureincluding Evolved Node Bs (ENBs) 120, 122, 124 and an anchor node 130,or ENBs 126 and 128 and an anchor node 132. A User Equipment (UE) 101 isconnected to an Internet Protocol (IP) network 114 via the E-RAN 110 or112. The ENBs 120 to 128 correspond to legacy Node Bs in the UMTS systemand are connected to the UE 101 via radio channels.

Compared to the legacy Node Bs, the ENBs 120 to 128 play a more complexrole. Since all user traffic including real-time service such as Voiceover IP (VoIP) is serviced on shared channels in the 3GPP LTE, an entityfor collecting the status information of UEs and scheduling them isrequired and the ENBs 120 to 128 are responsible for the scheduling.Generally, an ENB controls a plurality of cells.

The ENBs 120 to 128 also perform Adaptive Modulation and Coding (AMC) byselecting a modulation scheme and a channel coding rate for a UEadaptively according to the channel status of the UE. As with High SpeedDownlink Packet Access (HSDPA) of UMTS and High Speed Uplink PacketAccess (HSUPA) (or Enhanced Dedicated CHannel (EDCH)), the LTE systemuses Hybrid Automatic Repeat reQuest (HARQ) between the ENBs 120 to 128and the UE 101. Considering that a variety of Quality of Service (QoS)requirements cannot be fulfilled with HARQ alone, a high layer mayperform an outer ARQ between the UE 101 and the ENBs 120 to 128. HARQ isa technique for increasing reception success rate by soft-combiningprevious received data with retransmitted data without discarding theprevious data. High-speed packet communication systems such as HSDPA andEDCH use HARQ to increase transmission efficiency. To realize a datarate of up to 100 Mbps, it is expected that the LTE system will adoptOrthogonal Frequency Division Multiplexing (OFDM) in a 20-MHz bandwidthas a radio access technology.

FIG. 2 is a diagram illustrating a conventional contention detectionoperation during a Random Access (RA) procedure. Reference numeral 201denotes a UE and reference numeral 203 denotes an ENB.

Referring to FIG. 2, the UE 201 selects one of predetermined codesequences called RA Preambles randomly or in a predetermined rule andtransmits the selected RA Preamble on an asynchronous RA CHannel (aRACH)in step 211. In step 213, the ENB 203 transmits an RA Preamble responsemessage including an RA Preamble Identifier (id or index) for the RAPreamble, Timing Advance (TA) information by which to adjust UpLink (UL)timing synchronization, Grant information indicating UL resourcesallocated for transmitting Layer 2/Layer 3 (L2/L3) messages, and aTemporary Radio Network Temporary ID (T-RNTI) being a temporary UE ID.After receiving the RA Preamble response message, the UE 201 checks theRA Preamble id and if the checked RA Preamble id is identical to that ofthe transmitted RA Preamble, it transmits an L2/L3 message in the uplinkresources in step 221.

If a plurality of UEs transmit the same preamble to the ENB 203,contention occurs in the RA procedure. To notify the UE 201 of thesuccessful reception of the RA Preamble, the ENB 203 schedules aContention Resolution (CR) message including an ID specific to the UE201 or a random ID received from the UE 201 using the T-RNTI andtransmits it to the UE 201 in step 223. For this purpose, the UE 201includes its unique ID or a smaller random ID in the L2/L3 message ofstep 221.

Each UE having a T-RNTI can determine from the ID included in the CRmessage whether it has won or lost the RA contention. If the CR messagescheduled using the T-RNTI does not include the unique ID or random IDof the UE 201, the UE 201 re-starts the RA procedure, considering thatit has lost the contention.

The use of the CR message clearly indicates to a user as to whether ithas won or lost an RA contention. However, since a CR message need to betransmitted on the DownLink (DL) for each T-RNTI at each RA in theconventional procedure, signaling overhead is increased in terms ofoverall system performance.

SUMMARY OF THE INVENTION

An aspect of the present invention is to address at least the problemsand/or disadvantages and to provide at least the advantages describedbelow.

Accordingly, an aspect of the present invention is to provide a methodand apparatus for enabling a UE to detect a contention, while minimizingtransmissions of a CR message and thus reducing signaling overheadduring an RA procedure.

In accordance with an aspect of the present invention, a method isprovided for performing a random access procedure in a mobilecommunication system. The method includes receiving a random accessresponse message from a Base Station (BS); transmitting a messageincluding a Cell-Radio Network Temporary Identifier (C-RNTI) to the BS,if the C-RNTI exists; and determining that a contention for the randomaccess procedure is resolved successfully, when User Equipment(UE)-specific control information addressable by the C-RNTI is receivedfrom the BS.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is an exemplary view illustrating the configuration of a 3GPP LTEsystem;

FIG. 2 is a diagram illustrating a conventional contention detectionoperation during an RA procedure;

FIG. 3 is a diagram illustrating a contention detection operation duringan RA procedure according to an embodiment of the present invention;

FIG. 4 is a flowchart of an operation of an ENB according to anembodiment of the present invention;

FIG. 5 is a block diagram of an ENB apparatus according to an embodimentof the present invention;

FIG. 6 is a flowchart of an operation of a UE according to an embodimentof the present invention; and

FIG. 7 is a block diagram of a UE apparatus according to an embodimentof the present invention.

Throughout the drawings, the same drawing reference numerals will beunderstood to refer to the same elements, features and structures.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The matters defined in the description such as a detailed constructionand elements are provided to assist in a comprehensive understanding ofcertain embodiments of the present invention. Accordingly, those ofordinary skill in the art will recognize that various changes andmodifications of the embodiments described herein can be made withoutdeparting from the scope and spirit of the invention. Also, descriptionsof well-known functions and constructions are omitted for clarity andconciseness.

While the present invention will be described in the context of a 3GPPLTE system evolved from a 3GPP UMTS system, it is understood that thepresent invention is applicable to mobile communication systems with asimilar channel structure.

In accordance with an embodiment of the present invention, an ENBtransmits a CR message only when an L2/L3 message received from a UE isan initial access message in an RA procedure. If the L2/L3 messagetransmission is successful, the UE starts a timer. When the UE detectscontrol information on a DL channel, which is mapped to or includes UEidentification information being at least one of a Cell-RNTI (C-RNTI)that identifies the UE in a cell and a scrambling code before expirationof the timer, or when the UE receives a CR message including a unique IDof the UE or a random ID transmitted in the L2/L3 message, scheduledbased on a T-RNTI set in an RA Preamble response message beforeexpiration of the timer, the UE stops the timer and continues anon-going RA procedure. The control information includes at least one ofscheduling information, ACKnowledge/Negative ACKnowledge (ACK/NACK)information, and a C-RNTI-specific Cyclic Redundancy Check (CRC). On theother hand, if the timer expires without receiving either of the controlinformation and the CR message, the UE re-starts the RA procedure.

That is, the ENB allows message transmission through scheduling to a UEhaving a T-RNTI that the ENB transmitted in an RA Preamble responsemessage. However, when an RA contention occurs, a plurality of UEs mayhave the same T-RNTI. Thus, to indicate the UE to which the ENB wants totransmit a CR message, it includes a unique ID of the UE or a random IDreceived from the UE in an L2/L3 message in the CR message. Herein,scheduling a CR message based on a T-RNTI means that the CR message istransmitted in resources indicated by scheduling information includingthe T-RNTI.

If the UE already has UE identification information such as a C-RNTI ora scrambling code, it can detect control information on a DL controlchannel in many ways. For example, the UE senses transmission of thecontrol information for the UE on the DL control channel when: i)scheduling information uses a CRC mapped to the C-RNTI of the UE (i.e. aC-RNTI-specific CRC) and the UE detects no errors in the schedulinginformation received on the DL control channel by a CRC check using theC-RNTI, ii) an ACK/NACK for a transmitted UL message is encoded with thescrambling code allocated to the UE and transmitted to the UE and the UEdetects the ACK/NACK using its scrambling code on the downlink controlchannel, or iii) the UE receives a control command including an explicitthe C-RNTI of the UE on the DL control channel. In the presentinvention, control information means any kind of control informationthat can be signaled on a DL control channel.

FIG. 3 is a diagram illustrating a contention detection operation duringan RA procedure according to an embodiment of the present invention.Reference numeral 301 denotes a UE and reference numeral 303 denotes anENB.

Referring to FIG. 3, the UE 301 selects one of predetermined RAPreambles randomly or in a predetermined rule and transmits the selectedRA Preamble on a predetermined RA CHannel (RACH) in step 311. In step313, the ENB 303 transmits an RA Preamble response message including anRA Preamble ID (or index) for the RA Preamble, TA information by whichto adjust UL timing synchronization, Grant information indicating ULresources allocated for transmitting L2/L3 messages, and a T-RNTI beinga temporary UE ID.

After receiving the RA Preamble response message, the UE 301 determineswhether it already has a valid C-RNTI before the RA procedure in step321. The C-RNTI is an ID allocated to a connected-mode UE for use in acell. The C-RNTI is used to identify the UE during scheduling. The UEtransmits an L2/L3 message using the Grant information in step 331. Ifthe UE has the C-RNTI, it includes the C-RNTI in the L2/L3 message.Without the C-RNTI, the UE includes its unique ID such as a Packet-TMSI(P-TMSI) or an International Mobile Subscriber ID (IMSI) in the L2/L3message. While not shown, the UE may include a random ID generated bythe UE in the L2/L3 message, instead of the unique ID.

If the L2/L3 message transmission is successful, the UE 301 starts atimer in step 323. That is, the timer starts when the UE receives an ACKfor the L2/L3 message from the ENB 303. It can be further contemplatedas another embodiment of the present invention that the timer startswhen the L2/L3 message is transmitted. It can be further contemplated asa third embodiment of the present invention that the timer starts whenthe UE 301 receives the RA Preamble response message or transmits the RAPreamble.

In step 333, the ENB 303 determines whether the L2/L3 message is aninitial access message. An initial access message is a message that anidle-mode UE transmits to transition to a connected-mode. In general,the UE 301 does not have a valid C-RNTI when starting the RA procedure,the ENB 303 determines whether the UE 301 has the valid C-RNTI whenstarting the RA procedure by checking whether the L2/L3 message is aninitial access message. If the L2/L3 message is an initial accessmessage, the ENB 303 transmits a CR message scheduled based on a T-RNTIallocated to the UE 301, considering that the UE 301 does not have thevalid C-RNTI in step 335. The CR message includes the unique ID of theUE or the random ID received in the L2/L3 message.

On the contrary, if the L2/L3 message is not an initial access message,the ENB 303 transmits control information using the C-RNTI on a DLcontrol channel to the UE 301, considering that the UE 301 already hasthe valid C-RNTI in step 337. For example, the ENB 303 attaches a CRCmapped to the C-RNTI to a scheduling command to be transmitted to theUE, transmits a control command explicitly including the C-RNTI, ortransmits an ACK/NACK encoded with the scrambling code allocated to theUE 301 for the received L2/L3 message.

When the UE 301 senses any control information destined for the UE 301on a DL control channel or receives a CR message scheduled based on theT-RNTI and including the unique ID before expiration of the timer, itstops the timer and continues the on-going RA procedure, consideringthat it has won a contention that may occur during the RA procedure.

Meanwhile, if the UE 301 neither senses any control information destinedfor the UE 301 on the DL control channel nor receives a CR messagescheduled based on the T-RNTI and including the unique ID until theexpiration of the timer, it discontinues the on-going RA procedure andre-starts an RA procedure, considering that it has lost a contentionthat may occur during the RA procedure in step 341.

FIG. 4 is a flowchart of an operation of the ENB according to anembodiment of the present invention.

Referring to FIG. 4, the ENB receives an L2/L3 message on anasynchronous RACH (aRACH) from a UE in step 401. The L2/L3 message is amessage that the UE transmits in UL resources allocated by an RAPreamble response message. In step 411, the ENB determines whether theL2/L3 message is an initial access message. In a modified embodiment ofthe present invention, the ENB may determine whether the L2/L3 messageincludes a valid C-RNTI that the UE has before an RA procedure.

If the L2/L3 message is an initial access message, the ENB transmits tothe UE a CR message that is scheduled based on a T-RNTI allocated to theUE in the RA Preamble response message and that includes a unique ID ora random ID of the UE set in the L2/L3 message, considering that the UEdoes not have the C-RNTI in step 413. If the L2/L3 message is not aninitial access message, the ENB considers that the UE has the validC-RNTI and transmits control information on a DL control channel to theUE before expiration of a timer of the UE without transmitting the CRmessage in step 415. The control information includes controlinformation encoded with the C-RNTI and a scrambling code allocated tothe UE, or a CRC based on the C-RNTI.

FIG. 5 is a block diagram of an ENB apparatus according to an embodimentof the present invention.

Referring to FIG. 5, a transceiver 511 is responsible for transmissionand reception of a radio signal between the ENB and the UE. Upon receiptof an L2/L3 message from the transceiver 511, a message analyzer 521determines whether the L2/L3 message is an initial access message. Ifthe L2/L3 message is an initial access message, a CR message generator531 generates a CR message. A scheduler 551 schedules the CR messageusing a T-RNTI allocated to the UE by an RA Preamble response messageand transmits the CR message to the UE through the transceiver 511. TheCR message includes a unique ID or a random ID of the UE included in theL2/L3 message.

If the L2/L3 message is not an initial access message, the CR message isnot transmitted. A control information generator 541 generates controlinformation specific to the UE before expiration of a timer of the UEand transmits the control information to the UE on a DL control channelthrough the transceiver. The control information includes at least oneof scheduling information corresponding to the C-RNTI and the scramblingcode of the UE, an ACK/NACK, and a C-RNTI-specific CRC.

FIG. 6 is a flowchart of an operation of the UE according to anembodiment of the present invention.

Referring to FIG. 6, the UE receives an RA Preamble response message foran RA Preamble transmitted on an aRACH from the ENB in step 601 anddetermines whether it already has a valid C-RNTI before starting an RAprocedure in step 603. In the presence of the valid C-RNTI, the UEincludes C-RNTI in an L2/L3 message and transmits the L2/L3 message inUL resources allocated by the RA Preamble response message in step 611.On the other hand, in the absence of the valid C-RNTI, the UE generatesan L2/L3 message (i.e. an initial access message) including a unique IDof the UE or a random ID generated by the UE and transmits the L2/L3message in the UL resources allocated by the RA Preamble responsemessage in step 641.

In step 613, the UE starts a timer when it transmits the L2/L3 messageor receives an ACK for the L2/L3 message from the ENB. The UE monitorsexpiration of the timer 621. If the timer is still running, the UE goesto step 623.

In step 623, the UE monitors whether any control information for the UEhas been received on a DL control channel before the expiration of thetimer, or a CR message has been received which was scheduled based on aT-RNTI acquired from the RA Preamble response message and includes theUE ID transmitted in step 641. If at least one of the controlinformation and the CR message has been received, the UE stops the timerand performs a procedure corresponding to the received message in step631. If neither the control information nor the CR message has beenreceived until the expiration of the timer, the UE re-starts an RAprocedure, re-transmits an RA Preamble, and awaits reception of an RAPreamble response message in step 633. While not shown, upon receipt ofan RA Preamble response message for the retransmitted RA Preamble, theUE re-starts from step 601.

FIG. 7 is a block diagram of a UE apparatus according to an embodimentof the present invention.

Referring to FIG. 7, a transceiver 711 transmits and receives data andcontrol information to and from the ENB. Upon receipt of an RA preambleresponse message through the transceiver 711, a first UE ID manager 721extracts a T-RNTI from the RA Preamble response message and processesit. If the first UE ID manager 721 has a valid C-RNTI when the RAPreamble response message is received, a UL L2/L3 message generator 731generates an L2/L3 message and includes the C-RNTI in the L2/L3 message.In the absence of the valid C-RNTI when the RA Preamble response messageis received, a unique ID or a random ID of the UE processed in a secondUE ID manager 741 is included in the L2/L3 message. The second UE IDmanager 741 processes the unique ID such as a P-TMSI or an IMSI. Whenneeded, the second UE ID manager 741 generates the random ID andprovides it to the L2/L3 message generator 731.

The L2/L3 message is transmitted through the transceiver 711 and uponreceipt of an ACK for the L2/L3 message from the ENB, a timer 751starts. An L2/L3 message analyzer 761 monitors whether any controlinformation for the UE has been received on a DL control channel, or aCR message has been received that was scheduled based on the T-RNTI andincludes the unique ID or the random ID. If neither the controlinformation nor the CR message has been received until the expiration ofthe timer, an RA procedure is re-started.

As is apparent from the above description, the present inventionadvantageously enables efficient contention detection, thereby reducingsignaling overhead in an RA procedure.

While the invention has been shown and described with reference tocertain exemplary embodiments of the present invention thereof, it willbe understood by those skilled in the art that various changes in formand details may be made therein without departing from the spirit andscope of the present invention as defined by the appended claims andtheir equivalents.

1. A method for performing a random access procedure in a mobilecommunication system, the method comprising: receiving a random accessresponse message from a Base Station (BS); transmitting a messageincluding a Cell-Radio Network Temporary Identifier (C-RNTI) to the BS,if the C-RNTI exists; and determining that a contention for the randomaccess procedure is resolved successfully, when User Equipment(UE)-specific control information addressable by the C-RNTI is receivedfrom the BS.